1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to improving cell reselection.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications.
For example, in some current cell reselection methods, such as the methods used in WCDMA-to-WCDMA (W2W) intra-frequency reselection scenarios where a user equipment (UE) is in an idle mode or CELL_FACH, CELL_PCH, or other 3GPP-defined UE state, the UE is often configured to allow cell reselection from a current serving cell to a detected cell that is not included in a neighbor cell list maintained and communicated by the current serving cell in a system information block (SIB) message. Typically, reselection to these non-NCL detected cells is only permitted if the signal-to noise ratio (e.g. Ec/Io) associated with the current serving cell Qqualmeas is higher than a minimum allowable signal quality value Qqualmin.
But even where such non-NCL detected cell reselection is allowed, the signal quality of these detected cells are often ranked against those of the NCL cells before determining to which cell the UE should be reselected. Most often, this ranking is performed by computing a ranking value for each NCL and non-NCL cell according to a ranking value function, which includes a reselection weighting parameter, Qoffset. This offset parameter serves to weigh a ranking value toward or against reselection to any particular cell. For NCL cells, the Qoffset value is determined, managed, and/or communicated to the UE by the serving cell via the SIB message, such as the SIB11 (or SIB12 when the UE is in a CELL_FACH state). However, the Qoffset values of any detected cells not in the NCL are not defined in the SIB11 transmitted by the serving cell. Thus, in scenarios where detected cells must be ranked against NCL cells before a reselection decision is made, the UE must somehow obtain a Qoffset value for the detected cells to evaluate the detected cell ranking values before ranking and reselection may be performed.
According to some legacy cell reselection methods, UEs or networks set the Qoffset value of any non-NCL cell to zero where the appropriate Qoffset value is not included in the received SIB11. However, because the Qoffset value of a cell is inversely proportional to the likelihood that the cell is selected for reselection and NCL cells often have a positive Qoffset value in the SIB 11, these legacy cell reselection methods favor reselection toward non-NCL detected cells.
In other words, a UE that utilizes these legacy methods may prefer reselection to non-NCL cells over NCL cells. This may become problematic, however, as detected non-NCL cells often exhibit low signal quality and lower communication capability as compared to network-preferred NCL cells, and reselection to these cells may result in call drops and degradation in user experience.
Moreover, the problem gets worse if one or more femtocells are present in the vicinity of the UE. In traditional wireless systems, the network commands UEs to favor reselection to femtocells by setting each femtocell Qoffset to zero. Thus, if all non-NCL detected cell Qoffset values are set to zero, UEs and networks lose the advantage of using a cell Qoffset value to distinguish between macro cells and femtocells.
Thus, there is a need for a method and apparatus for improved cell reselection.